flat roof

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constructing a warm roof will not be possible due to the height of the roof.

small joists prevent a cold roof.

so i was wondering if i could have an empty void with no insulation, moisture plasterboard if it helps.

I understand that i may need ventilation, and i would do this via fascia board vents is this right?

if there is anything else i may need could you let me know

thanks
 
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If you are building this under a building notice, your inspector will take a particularly dim view of your plan.
 
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If you are building this under a building notice, your inspector will take a particularly dim view of your plan.
well warm roof will bulk it out about a foot, and cold roof you have to leave a gap, can’t remember how much. I can easily add vents on fascia board though.
 
well warm roof will bulk it out about a foot, and cold roof you have to leave a gap, can’t remember how much. I can easily add vents on fascia board though.
You could do a hybrid, or inverted roof. There is a thread a bit further down.

Basically insulation pushed up to underside of roof, level with bottom of joists, no gap, and a vapour
membrane across, then pb.

If you are getting it signed off, they won't let you not put insulation in.
 
constructing a warm roof will not be possible due to the height of the roof.

small joists prevent a cold roof.

so i was wondering if i could have an empty void with no insulation, moisture plasterboard if it helps.

I understand that i may need ventilation, and i would do this via fascia board vents is this right?

if there is anything else i may need could you let me know

thanks

What is the roof size and span

maybe you could put in 2 steels at 1 third and 2 thirds, then say 4 x 2 joists and insulation over the top
 
You could do a hybrid, or inverted roof. There is a thread a bit further down.

Basically insulation pushed up to underside of roof, level with bottom of joists, no gap, and a vapour
membrane across, then pb.

If you are getting it signed off, they won't let you not put insulation in.
Hi i did put a post on the roofers section, and i said i would do a cold roof with 50mm insulation flush with the bottom of the joists, this leaves a 50mm air gap that i will ventilate with fascia vents on 3 sides, on the bottom of the joists i will put a vapour barriers and top it off with a vapour plasterboard. Does that seem okay to you or do i need to make any adjustments? Thanks
 
What is the roof size and span

maybe you could put in 2 steels at 1 third and 2 thirds, then say 4 x 2 joists and insulation over the top
what do you mean by two steels? how far thick will this make the ceiling?

if you read what i said to mikey just then, do you think that will be okay? Cheers
 
Hi i did put a post on the roofers section, and i said i would do a cold roof with 50mm insulation flush with the bottom of the joists, this leaves a 50mm air gap that i will ventilate with fascia vents on 3 sides, on the bottom of the joists i will put a vapour barriers and top it off with a vapour plasterboard. Does that seem okay to you or do i need to make any adjustments? Thanks

yes it’s possible - cold roofs suffer condensation if ventilation is not good enough.


I’m not sure what insulation thickness would be - maybe 150mm


Why is height a problem if it’s a warm roof? - is there a low 1st floor window or planning height restriction
 
yes it’s possible - cold roofs suffer condensation if ventilation is not good enough.


I’m not sure what insulation thickness would be - maybe 150mm


Why is height a problem if it’s a warm roof? - is there a low 1st floor window or planning height restriction
inside floor of house is about 15cm higher. need to match up with this, also house has a pitched roof and we want the height of the extension to be at the last slate tile, so we can have the guttering same level, any higher will have a flat roof sticking out and will make the side profile look a bit weird. 4 inch is 100mm so hoping for 50mm insulation. Also it is going to be a bathroom so is going to have an extractor fan.
 
span of 4x2 from wall plate to wall plate is about 2 metres.
That span only needs 4 x 2 anyway.

warm roof would be 95mm + 11mm OSB + 120mm insulation + 18mm OSB then add on firrings if joists are set plumb

is that too much roof thickness, 245mm
 
inside floor of house is about 15cm higher. need to match up with this, also house has a pitched roof and we want the height of the extension to be at the last slate tile, so we can have the guttering same level, any higher will have a flat roof sticking out and will make the side profile look a bit weird. 4 inch is 100mm so hoping for 50mm insulation. Also it is going to be a bathroom so is going to have an extractor fan.

unfortunately the minimum flat roof insulation is 120mm for a warm roof to achieve 0.18 u value.

cold roof is more


Your only other option is to get a SAPs calculation for the whole house and prove any other house improvements done at the same time ( or at least recently) add up to the notional elemental value of the extension.
 

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